POLYMERIZATION TECHNIQUES 
        
       Polystyrene was first made by E. Simon in 1839 who at the time believed he had produced an 
       oxidation product, which he called styrol oxide. Since that time the polymerisation of styrene has 
       been extensively studied. In fact a great deal of the work which now enables us to understand the 
       fundamentals of polymerization was carried out on styrene. 
       The polymer may be prepared by mass, suspension, solution, and emulsion methods, the first two 
       being the most important. Mass polymerisation has the advantage of apparent simplicity and 
       gives a polymer of high clarity and very good electrical insulation characteristics. There are, 
       however, severe problems due to the exothermic reaction and the product has a broad molecular 
       weight distribution. Polymerisation in solution reduces the exothenn but may lead to problems of 
       solvent recovery and solvent hazards. The solvent may also act as a chain transfer agent and 
       cause a reduction in molecular weight. Suspension polymerisation avoids most of these problems 
       but there is some contamination of the polymer by water and the suspension agent. Furthermore 
       the polymer must be dried and aggregated before being sold as pellets suitable for injection 
       moulding and extrusion. Emulsion polymerisation techniques are seldom used with polystyrene 
       since  the  large  quantities  of  soap  used  seriously  affects  clarity  and  electrical  insulation 
       characteristics. This process is therefore used only for the production of polystyrene latex. 
        
       1-Bulk (Mass) Polymerization 
       Bulk or mass polymerization of a pure monomer offers the simplest process with a minimum of 
       contamination of the product. However, bulk polymerization is difficult to control because of the 
       characteristics  of  radical  chain  polymerization.  Their  highly  exothermic  nature,  the  high 
       activation  energies  involved,  and  the  tendency  toward  the  gel  effect  combine  to  make  heat 
       dissipation difficult. Bulk polymerization requires careful temperature control. Further, there is 
       also  the  need  for  strong  and  elaborate  stirring  equipment  since  the  viscosity  of  the  reaction 
       system increases rapidly at relatively low conversion. The viscosity and exotherm effects make 
       temperature  control  difficult.  Local  hot  spots  may  occur,  resulting  in  degradation  and 
       discoloration of the polymer product and a broadened molecular weight distribution due to chain 
       transfer to polymer. In the extreme case, uncontrolled acceleration of the polymerization rate can 
       lead  to  disastrous  ‘‘runaway’’  reactions  [Sebastian  and  Biesenberger,  1979].  Bulk 
       polymerization is not used commercially for chain polymerizations nearly as much as for step 
       polymerizations because of the difficulties indicated. It is, however, used in the polymerizations 
       of ethylene, styrene, and methyl methacrylate. The heat dissipation and viscosity problems are 
       circumvented  by  carrying  out  the  polymerizations  to  low  conversions  with  separation  and 
       recycling of unreacted monomer. An alternative is to carry out polymerization in stages—to low 
       conversion in a large reactor and to final conversion in thin layers (either on supports or free-
       falling streams). 
        
       2-Solution Polymerization 
       Polymerization of a monomer in a solvent overcomes many of the disadvantages of the bulk 
       process. The solvent acts as diluent and aids in the transfer of the heat of polymerization. The 
       solvent  also  allows  easier  stirring,  since  the  viscosity  of  the  reaction  mixture  is  decreased. 
       Thermal control is much easier in solution polymerization compared to bulk polymerization. 
       On the other hand, the presence of solvent may present new difficulties. Unless the solvent is 
       chosen with appropriate consideration, chain transfer to solvent can become a problem. Further, 
       the purity of the polymer may be affected if there are difficulties in removal of the solvent. Vinyl 
       acetate, acrylonitrile, and esters of acrylic acid are polymerized in solution. 
       By  polymerising  styrene  in  solution  many  problems  associated  with  heat  transfer  and  the 
       physical movement of viscous masses are reduced, these advantages being offset by problems of 
       solvent recovery and the possibility of chain transfer reactions. In 1955 Distrene Ltd started a 
       plant at Barry in South Wales for the production of styrene by such a solution polymerisation 
       process and some details have been made a ailable .The essential details of this process are 
       indicated by Styrene and solvent are blended together and then pumped to the top of the first 
       reactor which is divided into three heating zones. In the first zone the solution is heated to start 
       up the polymerisation reaction but because of the exothermic reaction in the second and third 
       zones of the first reactor and the three zones of the second reactor Dowtherm cooling coils are 
       used to take heat out of the system. By the time the reaction mixture reaches the third reactor the 
       polymerisation reaction has started to slow down and so the reaction mixture is reheated. From 
       the third reactor the polymer is then run into a devolatilising (‘stripping’) vessel in the form of 
       thin  strands.  At  a  temperature  of  225°C  the  solvent,  residual  monomer  and  some  very  low 
       molecular weight polymers are removed, condensed and recycled. The polymer is then fed to 
       extruder units, extruded as filaments, granulated, lubricated and stored to await dispatch. 
        
       3- Suspension Polymerization 
       The average molecular weight of most bulk polymerised poly(methy1 methacrylates) is too high 
       to give a material which has adequate flow properties for injection moulding and extrusion. By 
       rolling  on  a  two-roll  mill  the  molecular  weight  of  the  polymer  can  be  greatly  reduced  by 
       mechanical scission, analogous to that involved in the mastication of natural rubber, and so 
       mouldable  materials  may  be  obtained.  However,  bulk  polymerisation  is  expensive  and  the 
       additional milling and grinding processes necessary make this process uneconomic in addition to 
       increasing  the  risk  of  contamination.  As  a  result  the  suspension  polymerisation  of  methyl 
       methacrylate was developed to produce commercial material such as Diakon made by ICI. Such 
       a polymerisation can be carried out rapidly, usually in less than an hour, because there is no 
       serious exotherm problem. There is, however, a problem in controlling the particle size of the 
       beads formed and further in preventing their agglomeration, problems common to all suspension-
       type polymerisations. The particle size of the beads is determined by the shape and size of the 
       reactor, the type and rate of agitation and also the nature of suspending agents and protective 
       colloids  present.  Suspending  agents  used  include  talc,  magnesium  carbonate  and  aluminium 
       oxide  whilst  poly(viny1  alcohol)  and  sodium  polymethacrylate  are  among  materials  used  as 
       protective colloids. 
       In one process described in the literature’ one part of methyl methacrylate was agitated with two 
       parts  of  water  and  0.2%  benzoyl  peroxide  was  employed  as  the  catalyst.  Eight  to  18  g  of 
       magnesium carbonate per litre of reactants were added, the lower amount being used for larger 
       beads, the larger for small beads. The reaction temperature was 80°C initially but this rose to 
       120°C because of the exothermic reaction. Polymerisation was complete in about an hour. The 
       magnesium carbonate was removed by adding sulphuric acid to the mixture. The beads were 
       then filtered off, carefully washed and dried. 
       Other  additives  that  may  be  incorporated  include  sodium  hydrogen  phosphates  as  buffering 
       agents to stabilise that pH of the reaction medium, lauryl mercaptan or trichlorethylene as chain 
       transfer agents to control molecular weight, a lubricant such as stearic acid and small amounts of 
       an  emulsifier  such  as  sodium  lauryl  sulphate.  The  dried  beads  may  be  supplied  as  injection 
       moulding material without further treatment or they may be compounded with additives and 
       granulated. 
        
       Suspension  polymerisation  of  styrene  is  widely  practised  c~mmercially.I~n  this  process  the 
       monomer is suspended in droplets $-$in. in diameter in a fluid, usually water. The heat transfer 
       distances for the dissipation of the exotherm are thus reduced to values in the range &-&in. 
       Removal of heat from the lowviscosity fluid medium presents little problem. The reaction  is 
       initiated by monomer-soluble initiators such as benzoyl peroxide. 
        
       It  is  necessary  to  coat  the  droplets  effectively  with  some  suspension  agent,  e.g.  poly(viny1 
       alcohol), talc etc., to prevent them cohering. Control of the type and quantity of suspension agent 
       and of the agitation has a pronounced effect on the resulting particles. It is not unknown for the 
       whole of the polymerising mass to aggregate and settle to the bottom of the reaction vessel 
       because of such conditions being incorrect. Following polymerisation, unreacted monomer may 
       be removed by steam distillation and the polymer is washed and dried. 
       The disadvantages of the suspension process are that about 70% of the volume of the kettle is 
       taken up by water, the need for a drying stage which could cause discolouration by degradation 
       and  the  need  to  convert  the  small  spheres  formed  into  a  larger  shape  suitable  for  handling. 
       Furthermore, the suspension method cannot easily be converted into a continuous process. 
        
       4- Emulsion Polymerization 
       Because of the  large  quantities  of  soap  left  in  the  polymer,  which  adversely  affects  clarity, 
       electrical insulation characteristics and problems in agitation and densification, this process is 
       used only for making latices. The techniques used are in many respects similar to those for 
       emulsion polymerised PVC. 
       Emulsion  polymerization  refers  to  a  unique  process  employed  for  some  radical  chain 
       polymerizations. It  involves the polymerization  of  monomers  in the  form of emulsions (i.e., 
       colloidal dispersions). The process bears a superficial resemblance to suspension polymerization 
        but is quite different in mechanism and reaction characteristics.